Fatigue and Stress Ratio

Ways of Applying Cyclic Stresses

• Materials can be subjected to cyclic stresses in three ways: axial, torsional and flexural. Imagine you have a rod of a particular material. Axial stresses are applied by compressing or stretching the rod lengthwise. Torsional stresses are applied by twisting each end of the rod in opposite directions. Flexural forces are applied by bending the rod into a U-shape.

Types of Cycling Stresses

• Cyclic stresses are classified as either repeated and reversed stresses or fluctuating stresses. In the former category, the mean stress and stress ratio equal zero because alternating the compressive and tensile stresses are equal and opposite in size. For the latter category, the mean stress and stress ratio do not equal zero.

Subcategories of Fluctuating Stress

• Fluctuating stress has three subcategories. The first case is when there is a tensile mean stress and a compressive stress is not applied. Since the maximum tensile stress is greater than the minimum tensile stress, the stress ratio is between zero and one. The second case is when there is a compressive mean stress and no tensile stress. Consequently, the stress ratio is greater than one. The third case is when there is a repetitive stress in only one direction. The minimum and maximum stresses are equal to and greater than zero respectively, so stress ratio is then equal to zero.

Stress, Fatigue and Failure

• These cycling stresses cause materials to fatigue and eventually break. Materials scientists call this failure. Failure happens suddenly but builds up in several stages. Firstly, stresses cause microstructural changes in the material. These lead to the creation of microscopic cracks that then grow and coalesce into larger cracks called striations. These cracks grow and cause the material to become unstable. Eventually, the material will suddenly break and fail. Fatigue testing uses S-N curves which plot stress versus cycles to failure